A body area network (BAN) is formed by using a plurality of terminal devices that are disposed on the surface of a human body, in the body, in the vicinity of the body, or in other spots. Attempts have been made to apply BANs to medical and health care fields, to support of disabled people, to creation of electronic entertainment devices such as a game controller, and to others.
SF1 in FIG. 1 illustrates an example of a superframe used in a BAN. The superframe illustrated in FIG. 1 includes a beacon, a data communication slot and a control slot. A data communication slot is used for communications between a base station and a terminal device that has established a communication with the base station. A beacon is used by a base station for reporting a timing for a synchronization signal and for an access to a terminal device that has established a communication. A control slot is a slot that a plurality of terminal devices can access on an as-needed basis, and is used for transmitting and receiving for example a notification signal and a control packet used for establishing communications between a base station and a terminal device.
In the example illustrated in FIG. 1, a base station transmits a notification signal when each control slot starts (arrow A1). In order to generate a connection to a base station that is in a good communication condition from among base stations nearby, a terminal device selects a connection destination from among base stations from which the terminal device succeeded in receiving a notification signal within a prescribed period. Determining a base station as a connection base station, the terminal device transmits a connection request to the base station determined as a connection base station (arrow A2). Receiving the connection request, the base station sets a period of performing a communication with the terminal device as the transmission source of the connection request to a period in a data communication slot, and thereafter transmits a connection response to the terminal device as a response to the connection request (arrow A3). In this example, the connection response contains information that specifies a timing for performing a communication between the base station and the terminal device having transmitted the connection request. Accordingly, the terminal device establishes a connection to the base station by using the connection response, and thereafter transmits a data packet that is addressed to the base station during the specified period in the data communication slot specified by the base station (arrow A4). Note that while the example of FIG. 1 has arrow A4 illustrating a case where a terminal device transmits data to a base station, transmission of data from the base station to the terminal device also starts after the establishment of a connection between the base station and the terminal device.
As a related art, a communication scheme is proposed in which a base station periodically confirms whether or not there is transmission of data from a terminal device to which a connection has already been established. According to this scheme, a terminal device transmits a packet containing a long preamble. The base station confirms part of the long preamble and thereafter receives data transmitted from the terminal device during a period other than a period during which the confirmation is performed periodically (for example, patent document 1 etc.). Also, a communication system including a base station and a plurality of grouped terminal devices has been invented. According to this system, an isolated time slot is assigned to a terminal device in the group, the phase is adjusted for each group, and communications between a terminal device for which the phase has been adjusted and the base station are conducted in a prescribed time slot (Patent Document 1 for example). A terminal device is also known that repeatedly transmits the same upstream user data to a base station the number of times determined on the basis of a value related to the distance to the base station (Patent Document 2 for example). Further, a broadcasting system is also known that uses a frame containing a movement reception symbol and a fixed reception symbol (Patent Document 3 for example).
Patent Document 1: Japanese Laid-open Patent Publication No. 9-294099
Patent Document 2: Japanese Laid-open Patent Publication No. 2009-130904
Patent Document 3: Japanese Laid-open Patent Publication No. 9-135230
Non-Patent Document 4: IEEE Std 802.15.4, IEEE Standard for Local and Metropolitan Area Network—Part 15.4: Low-Rate Wireless Personal Area Networks (LR-WPANs), [online], 27 Apr. 2012, IEEE Standard Association, [searched on Jul. 29, 2014], Internet <URL: http://standards.ieee.org/findstds/standard/802.15.4g-2012.html>
When data transmitted and received after the establishment of a connection is data for which instantaneous processes are desirable, the data sometimes fails to receive a process in time depending upon a time that it takes the terminal device to establish a connection by using the control slot between the terminal device and the base station as a communication destination. While a case of using a BAN has been explained as an example, a similar problem occurs also in communications using other networks. Also, because it is not possible to reduce a time taken to perform a process of starting transmission and reception of data such as the establishment of a communication between a terminal device and a base station no matter which of the related arts described above is applied, a long time is taken before emergency data starts to be processed.